This invention relates to optical recording-reproducing devices, and more particularly to multi-beam optical recording-reproducing devices by which a plurality of light beams are radiated on an information storage medium to effect recording, reproduction, and erasure of information thereon.
In the case of conventional optical recording-reproducing devices, a single laser light beam is radiated on an information storage medium, such that recording, reproduction, or erasure of information is effected by means of this single beam. However, for the purpose of enhancing the recording and reproduction efficiency, optical recording-reproducing devices have been proposed by which a plurality of beams are radiated on the information storage medium. The functions served by the plurality of beams include (a) real time monitoring, (b) overwriting on recorded information, and (c) parallel recording and reproduction.
In the real time monitoring (a), at least two beams are radiated on the same track on an information storage medium. The information is recorded by the leading beam and the recorded information is reproduced by the trailing beam immediately after the recording, so as to check whether the information has been recorded correctly. The recording of information and its reproduction for error checking purposes can thus be effected simultaneously. It is noted that in the case of a conventional single-beam device, the recorded information is reproduced for the error checking purposes after a full turn of the medium, which results in loss of time.
In the overwriting of information (b), as in the case of real time monitoring (a), at least two beams are radiated on the same track on the information storage medium. The information recorded on a re-writable information storage medium is erased by the leading beam, while the trailing beam records the new information. Thus, the erasure and recording can be effected simultaneously. It is noted that in the case of conventional signal beam device, the new information is recorded after a full turn of the medium after the old information is erased.
In the parallel recording and reproduction (c), a plurality of beams are radiated on several distinct tracks on the information storage medium, such that recording, reproduction, and erasure can be effected simultaneously. A large amount of information can thus be processed simultaneously.
FIG. 1 shows the overall organization of a conventional optical recording-reproducing device utilizing two laser beams for the real time monitoring purpose, which is disclosed, for example, in the collection of papers: "Optical Memory Symposium '85," pp. 107 through 112. In FIG. 1, a two-beam semiconductor laser 1 emits a recording beam 2 and a reproducing beam 3 parallel to each other (see FIG. 2 for details of the two-beam semiconductor laser 1). Although an array type two-beam semiconductor laser 1 having two active regions is shown in the figures, a semiconductor laser device consisting of two ordinary laser elements each having one active region disposed parallel to each other may also be utilized, provided that the recording beam 2 and reproducing beam 3 can be driven independently of each other.
Further, a collimator lens 4 is disposed at the beam-emitting side of the two-beams semiconductor laser 1, and a polarization beam splitter 5 receives the recording beam 2 and the reproducing beam 3 transmitted through the collimator lens 4. A reflection mirror 6 directs upward (the direction being as represented in FIG. 1) the recording beam 2 and reproducing beam 3 transmitted through the polarization beam splitter 5. Further, a 1/4-wavelength plate 7 and an objective lens 8 are disposed in the optical path between the reflection mirror 6 and the medium 9. The disk-shaped information storage medium 9 is disposed near the objective lens 8. Guide grooves 10 are formed along the information recording direction of the information storage medium 9, and a recording spot 11 and a reproduction spot 12 are formed on a track (formed between two adjacent grooves 10) via the recording beam 2 and reproducing beam 3, respectively, radiated on the information storage medium 9 along the guide grooves 10 (see FIG. 3 for detail).
To a side of the polarization beam splitter 5 is disposed a half mirror prism 13 which divides into reflected and transmitted portions the beams reflected by the polarization beam splitter 5. A two-partitioned photosensor 14 having two sensor surfaces 14a and 14b receives the beams transmitted through the half prism 13. A convex lens 15 converges the beams reflected by the half prism 13. A pin-hole mirror 16 for reflecting the recording beam 2 coming from the convex lens 15 has a pin hole 17 for selectively passing therethrough the reproducing beam 3. The reproducing beam 3 passing through the pin hole 17 is divided into reflected and transmitted beams by a half mirror prism 18. A knife-edge 19 is disposed on the optical path of the reproducing beam 3 transmitted through the half prism 18, and a two-partitioned photosensor 20 having a pair of sensor surfaces 20a and 20b receives the reproducing beam 3 via the knife-edge 19. On the other hand, a photosensor 21 receives the recording beam 2 reflected by the pin-hole mirror 16, thereby generating a monitoring signal E of the recording beam 2. Another photosensor 22 receives the reproducing beam 3 reflected by the half prism 18, thereby generating a reproduction output C. A reproduction signal detector circuit 23 generates in response to the reproduction output C a reproduction signal D (see FIG. 4).
On the other hand, a recording signal generation circuit 24 outputs a recording signal A in the form of a train of pulses (see FIG. 4). A driver circuit 25 drives the two-beams semiconductor laser 1 in response to the recording signal A.
A differential amplifier 26 for generating the tracking error signal TS has a pair of inputs for receiving the outputs of the sensor surfaces 14a and 14b of the photosensor 14. The two-partitioned photosensor 14 and the differential amplifier 26 constitute a well-known tracking error detection system known as the push-pull method system. A differential amplifier 27 for generating the focusing error signal FS has a pair of inputs for receiving the outputs of the sensor surfaces 20a and 20b of the photosensor 20. The knife-edge 19 and the two-partitioned photosensor 20 constitute a well-known focusing error detection system known as the knife-edge method system.
FIG. 3 shows the details of the recording spot 11 and the reproduction spot 12 formed on the information storage medium 9. The figure shows the case where the spots 11 and 12 are formed on a track formed between adjacent guide grooves 10. However, where preferred, the recording and reproduction may be effected on the guide grooves 10 themselves. In FIG. 3, the separation or distance between the recording spot 11 and the reproduction spot 12 is represented by reference character 1, while the rotational direction of the information storage medium 9 is indicated by an arrow. The pits 28 are formed (i.e., written) on the information storage medium 9 by the recording spot 11.
Next, the method of operation of the conventional optical recording-reproducing device of FIGS. 1 through 3 is described.
A recording signal A as shown in FIG. 4 is generated by the recording signal generation circuit 24, and the two-beam semiconductor laser 1 is driven by the driver 25 in accordance with the recording signal A. The recording beam 2 and the reproducing beam 3 emitted from the two-beam semiconductor laser 1 are collimated into parallel beams via the collimator lens 4, and after proceeding through the polarization beam splitter 5, reflection mirror 6, 1/4-wavelength plate 7, and the objective lens 8 are radiated on the information storage medium 9 to form a recording spot 11 and a reproduction spot 12 thereon.
The recording spot 11 contains the recording information (such as the pulse widths) of the recording signal A, and thus successively forms on the information storage medium 9 the pits 28 having forms B corresponding to the recording information (see FIG. 4). On the other hand, the reproduction spot 12, trailing behind the recording spot 11 at a distance 1 and driven at a predetermined weaker intensity of light, reproduces the information of the pits 28 after an interval of time t1 (about a few microseconds) corresponding to the distance 1.
Namely, the recording spot 11 successively forms pits 28 on the information storage medium 9 and is reflected therefrom. The reproduction spot 12 is reflected by the pits 28 written by the recording spot 11. The recording beam 2 and the reproducing beam 3 thus reflected by the information storage medium 9 are transmitted through the objective lens 8 and 1/4-wavelength plate 7. The direction of polarization of the beams 2 and 3 is rotated by 90 degrees when they thus pass through the 1/4-wavelength plate 7 forward and backward. Thus, the beams 2 and 3 reflected from the information storage medium 9 are reflected by the polarization beam splitter 5. Next, parts of the beams 2 and 3 are reflected by the half prism 13, while parts thereof are transmitted through the half prism 13 and are inputted to the tracking error detection optical system (described in detail hereinbelow) to be utilized for the correction of the tracking error of the beams radiated on the information storage medium 9.
On the other hand, the beams 2 and 3 reflected by the half prism 13 are converged by the convex lens 15. The recording beam 2 is thereafter reflected by the pin-hole mirror 16, while the reproducing beam 3 passes through the pin-hole 17 thereof and partially is reflected by the half prism 18. The part of the reproducing beam 3 transmitted through the half prism 18 is input to the focusing error detection optical system (described in detail hereinbelow) to be utilized therein for the correction of the focusing error of the beams radiated on the information storage medium 9.
The recording beam 2 reflected by the pin-hole mirror 16 is received by the photosensor 21 and is thereby detected in the form of a pulse-shaped waveform E corresponding to the recording signal A, to be utilized for the detection of obstacles which are present on the information storage medium 9 or in the optical paths of the system, etc.
On the other hand, the reproduction beam 3 reflected by the half prism 18 is received by the photosensor 22, and is thereby detected in the form of reproduction output C having a waveform corresponding to the forms B of pits. The waveform of the output C is further processed by the reproduction signal detection circuit 23 to be output therefrom as the reproduction signal D in the form of a pulse train. The thus obtained reproduction signal D is compared with the recording signal A, to judge whether recording of information has been effected correctly.
The above example shows the case where the reflectivity of the information storage medium 9 is decreased by the formation of the pits 28. The real time monitoring of the information recording state, however, can also be effected in a manner similar to the above where the reflectivity of the information storage medium 9 is increased by the pits 28 formed thereon. Further, it is noted that although the reproduction signal D is delayed with respect to the recording signal A, the delay time tl is on the order of several microseconds and hence the detection of the recording defects can be said to be effected substantially in the real time mode.
In the case where the information recorded on the information storage medium 9 is reproduced and no recording of information is effected, only the reproducing beam 3 is radiated from the two-beam semiconductor laser 1 and is detected by the reproduction signal detection circuit 23.
Next, referring to FIG. 5, the operation of the conventional focusing error detection optical system of the knife-edge method, constituted by the knife-edge 19, two-partitioned photosensor 20, and the differential amplifier 27, is described. It is noted that FIG. 5 shows only those portions which are essential to the understanding of the principles of operation. FIG. 5(a) shows the case where the information storage medium 9 is at the focal point of the objective lens 8, wherein the reproduction beam 3, formed into a semi-circular sectional form by the knife-edge 19, is focused on the center of the two-partitioned photosensor 20 just between the two light-receiving surfaces 20a and 20b. Under this circumstance, the output signal FS (focusing error signal) of the differential amplifier 27 is equal to zero. FIG. 5(b) shows the case where the information storage medium 9 is situated farther away from the objective lens 8 than at the focal point thereof, wherein the reproducing beam 3, partially interrupted by the knife-edge 19, is almost totally received by the lower (as represented in FIG. 5) light-receiving surface 20a, such that the output FS of the differential amplifier 27 becomes positive. On the other hand, FIG. 5(c) shows the case where the information storage medium 9 is nearer to the objective 8 than at the focal point thereof, wherein the reproducing beam 3 is incident on the upper light-receiving surface 20b, such that the output signal FS of the differential amplifier 27 becomes negative. Thus, a focusing error signal FS corresponding to the direction and magnitude of the focusing error is obtained via the differential amplifier 27.
Next, the method of operation of the conventional tracking error detection optical system of the push-pull method, constituted by the two-partitioned photosensor 14 and the differential amplifier 26, is described by reference to FIG. 6. It is noted that FIG. 6 shows only those portions which are essential to the understanding of the principles of the tracking error detection. FIG. 6(a) shows the case where there is no tracking error; FIG. 6(b) shows the case where the information-recording track is displaced to the positive direction of the X-axis (the direction+X); and FIG. 6(c) show the case where the track is displaced in the negative direction of the X-axis (the direction-X). The distributions of the first order diffraction light formed by the upper and lower edges of the track are represented by the curves 29 and 30, respectively. FIG. 6(d), (e), and (f) show the forms of the incident light spots on the two light-receiving surfaces 14a and 14b of the two-partitioned photosensor 14, with respect to the cases FIG. 6(a), (b), and (c), respectively. The recording beam 2 and the reproducing beam 3 form respective incident light spots having outer circular shapes on the photosensor 14. The centers of these incident light spots are positioned on the partition line between the two light-receiving surfaces 14a and 14b, being displaced from each other along the direction of the partition line. The distributions of the first order diffraction light 29 and 30 are shaded in FIG. 6(d) through (f).
When there is no tracking error as shown in FIG. 6 (a), the distributions of the first order diffraction lights 29 and 30 formed by the two edges of the track between the guide grooves 10 are equal to each other. Thus, as shown in (d), the output of the two light-receiving surfaces 14a and 14b of the photosensor 14 are equal to each other, such that the output TS (tracking error signal) of the differential amplifier 26 is equal to zero. When, on the other hand, the track between the guide grooves 10 is displaced in the direction+X, the diffraction light distribution 29 becomes greater than the distribution 30, such that the intensities of incident light spots on the photosensor 14 become unbalanced and a positive output signal TS is generated from the differential amplifier 26. When the track is displaced toward-X as shown in (c), the distribution of the diffraction light 30 become greater than the distribution 29, and the output signal TS of the differential amplifier 26 becomes negative. Thus, the tracking error signal TS corresponding to the direction and the magnitude of the tracking error is obtained via the differential amplifier 26.
The above conventional optical recording-reproducing device, however, has the following disadvantages. Since both the recording beam 2 and the reproducing beam 3 are received on the two-partitioned photosensor 14, it is impossible to detect the tracking errors of the recording beam 2 and the reproducing beam 3 independently. Thus, the precision and reliability of the tracking control are low. Further, since the focusing error is detected only for the reproducing beam 3, the focusing error of the recording beam 2 may become substantial. These disadvantages become particularly manifest when the alignment of the optical system become deteriorated with the passage of time. A further disadvantage is that the optical system is complicated.